The present invention relates to a process for eliminating low molecular-weight polymers from the recirculating gases of facilities carrying out ethylene polymerization at high pressure and high temperature.
Polymerization and copolymerization of ethylene at a pressure of 300 to 2500 bars and at a temperature of 130.degree. C. to 350.degree. C. are known reactions. In such facilities, the mixture of unpolymerized gaseous monomer(s) and liquid polymer leaving the reactor is first expanded into a separator operating at a pressure of 50 to 500 bars. These unpolymerized gaseous monomers shall be referred to hereinafter as recirculating gases. After the separator, these gases are cooled to a temperature on the order of 20.degree. to 60.degree. C., then returned to a compressor. The efficiency of the separator being imperfect, the recirculating gases contain polymer either in the form of condensed particles (solid or liquid) or in the form of products dissolved in said gases.
The polymer present in the recirculating gases in the form of condensed particles may come:
either from a vesicular carrying away of the polymer located in the separator,
or from a precipitation of the polymers initially dissolved, since their solubility in the recirculating gases is reduced by the progressive lowering of the temperature and pressure of these gases. The polymers present in the recirculating gases are generally low molecular-weight products called low polymers or fats.
The condensed particles of polymer present in the recirculating gases may be liquid or solid particles according to the temperature of the recirculating gas. In facilities carrying out production of high density polyethylene, the condensed particles are generally liquid when the gas temperature is higher than about 90.degree. C.; in facilities carrying out production of low density polyethylene, the condensed particles are generally liquid when the gas temperature is higher than about 60.degree. C.
It is well known in the high pressure polymerization technique that the progressive accumulation of low polymers in the gas recirculating lines causes clogging, prevents normal operating of the compressor, and intermittently involves considerable maintenance costs for the facility. The present invention therefore relates to a process for removing condensed polymer particles from the recirculating gases.